(1) Field of the Invention
The present invention relates to a control apparatus for an electroexpansive actuator and to a control apparatus and method for controlling a fuel injection valve which supplies fuel to a combustion chamber of an internal combustion engine by using the electroexpansive actuator.
(2) Description of the Related Art
An electroexpansive actuator (hereinafter referred to as the "actuator") generally comprises a group of piezoelectric elements. If a +500 V voltage is applied to the piezoelectric elements, the actuator expands, and if a -200 V voltage is applied to the elements, the actuator contracts. Therefore, in a conventional control apparatus for such an actuator, an amount of contraction of the actuator (i.e., the stroke) can be controlled only by a given constant amount.
When a fuel injection valve is controlled by such an actuator, the following problems are created.
In an internal combustion engine for a vehicle (more specifically, a gasoline engine), a control range for a desired amount of fuel is about 50 times a minimum amount of fuel to be supplied. Therefore, when the amount of fuel is controlled using a fuel injection valve which measures the amount of fuel with reference to a drive frequency, a maximum drive frequency must be about 50 times that of a minimum frequency.
This minimum frequency must be higher than an intake frequency corresponding to a maximum speed of the engine. For example, in a 4-cycle engine, the minimum frequency is 50 Hz or higher with respect to a maximum engine speed of 6,000 rpm. If the minimum frequency is set to 50 Hz, the drive frequency of the fuel injection valve must be within a control range of 50 Hz to 2,500 Hz.
However, even for a high-response fuel injection valve for the gasoline engine, the maximum stable frequency that can be achieved is limited to about 1,000 Hz because of a response-delay during the fuel intake operation. Accordingly, when the stroke of the actuator is constant, a wide control range of injection amount of fuel cannot be obtained.
Also, if such an actuator with a constant stroke is applied to a fuel injection valve for, e.g., a diesel engine, since the lift amount of a nozzle needle is predetermined, an opening area for the injection, and therefore, a fuel injection rate, becomes constant. As a result, the amount of fuel can be controlled only by a valve-open period. Therefore, when an operation range varies widely, as in a vehicle engine, this creates a decrease in power due to a prolonged fuel injection period during high-speed running, and conversely, creates engine noise and a degradation of the emission due to a shortened fuel injection period during low-speed running.
As an effective countermeasure, a known pilot injection operation, in which fuel injection is divided into two or more steps, can be performed. In this case, however, the fuel injection valve cannot be effectively controlled unless a miminum injection amount of fuel for the valve is below the amount that is necessary for appropriate pilot injection (i.e., about 1 mm.sup.3), thus requiring a very short response time.